For copper foils that have been conventionally used, a various kinds of copper foils are introduced to the market, and which copper foil should be used has been determined depending on the uses of printed-wiring boards. They include, for example, copper foils comprising nickel layers for forming resistors, heat-resisting copper foils to be used for sites that are exothermically affected in a direct manner by electronic equipment, and copper foils having excellent chemical resistance that are advantageously used for formation of fine pitch circuits.
In the trend in recent years toward miniaturization of electric equipment, miniaturization is also required for the printed-wiring board to be contained therein, and the formed copper foil circuit is further reduced in width. Furthermore, as computers operate faster, the processing speed is also enhanced, and clock frequencies are continuously increasing. Thus, for keeping up with improvement in performance of computer apparatuses, and achieving further miniaturization, provision of fine pitch circuits having increased wiring densities becomes essential.
As the wiring density of the printed-wiring board is increased and components implemented therein are further integrated, the amount of heat generation is increased, thus causing a problem. For example, the strength of bonding between the copper foil forming the circuit of the printed-wiring board and a substrate is reduced with time, and in some extreme cases, the copper foil circuit may be peeled off spontaneously from the base material. Therefore, current materials for printed-wiring boards are subjected to a variety of treatments to prevent problems before they happen.
The printed-wiring board can be considered as a composite product composed of a metal and a resin material, and thus improvement of its heat resistance will be influenced by a variety of factors such as the composition of the resin material and the type of surface treatment of the cupper foil. As copper foils having excellent heat resistance for printed-wiring boards, those having thick zinc layers or brass layers formed on nodular treatment sides thereof have been widely known. That is, the heat resistance with respect to the printed-wiring board generally refers to that of the product conforming to UL Standard. The thick zinc layer or brass layer provided on the nodular treatment side of the copper foil for ensuring conformation to UL Standard exhibits excellent performance to secure heat resistance.
On the other hand, in formation of small fine pitch circuits, the printed-wiring board comprising a 50-μm pitch signal transmission circuit with its circuit width of 25 μm and its inter-circuit gap of 25 μm has also commonly produced. Thin copper foils have been used in production of printed-wiring boards comprising such fine circuits because a satisfactory etching property is required when the copper foil is etched to form a circuit. Also, the additive method has been widely used in which an outer-layer copper foil is once completely etched away, and thereafter the copper foil circuit is formed by the plating method or the like.
However, for processing fine via holes and the like, laser drilling processing has been used in recent years, thus making it difficult to process the via hole with the copper foil bonded thereto, and therefore the conformal mask method in which the outer-layer copper foil is partially etched away to carry out laser drilling processing, the method in which the outer-layer copper foil for improving accuracy of position for drilling process is wholly etched away, and so on are employed. Then, after laser drilling processing is carried out, a copper layer is formed through the panel plating method and patterned to form a circuit in the site in which the copper foil has been etched away, or a copper foil circuit is directly formed by the additive method.
The problem arising in such methods is that after the copper foil is once removed, a surface treatment layer that would exist if the original copper foil were used does not exist in the interface between the circuit formed by the panel plating method or the additive method and the substrate. That is, absence of the surface treatment layer means that in the circuit portion is provided no means for purposely improving its chemical resistance and heat resistance.
Therefore, in particular, the heat resistance property of that circuit portion is significantly reduced compared to the case where a normal copper foil having improved heat resistance is used, and materials and methods preventing such a problem from arising have been desired.